1. Field of the Invention
This invention relates to a continuous single-stage high-speed spinning method for polyamidic multifilament yarns composed of specially suitable polymers for use as PA6 (Nylon 6) for the extrusion of molten polymer through dies and collection of the yarn at rates in the 3,500 to 5,500 meters/minute range, directly usable in standard textile operations without any further treatments.
2. Prior Art
It is known that multifilament yarns of PA6 are produced by means of different processing cycles using methods of the discontinuous or continuous types.
In general, in the instance of discontinuous methods, production takes place in two stages: spinning at a picking rate of about 800 to 1200 m/min followed by stretching, during which step the multifilament threads are stretched to values ranging from 3 to 4 times their initial lengths.
The stretching operation imparts the yarns with the desired final characteristics making it suitable for subsequent processing.
Such discontinuous methods normally require considerable involvement in terms of investments for the required equipment to implement them, and a high cost for the operation of such lines in terms of labor and effort.
In the instance of continuous methods, instead, a so-called combined spinning and stretching technique is used.
Compared to methods of the discontinuous type, extrusion through the dies take place at comparable values to those used in the discontinuous methods, but the stretching step takes place in a separate section from spinning, although in-line on the same equipment and at much higher rates than with the two-stage process.
With that equipment, the yarn picking speed is generally much higher than with the discontinuous process, and reaches levels on the order of 3,000 to 5,000 meters/minute.
Even in that case, however, the desired textile properties are imparted to the yarn by the stretching station, i.e. by the assembly comprising two or more spinning rollers normally intended for such an operation and always present in equipment of this type.
The presence of rotary members wherearound the yarn is passed is undesirable both as a way of reducing installation costs and because of the problems which arise in connection with the operation of such items, always costly, e.g. of the maintenance problems.
Single-stage continuous spinning processes have already been proposed at the rates contemplated by this invention and without stretching rollers, but the yarns obtained thereby are of the so-called POY (Partially Oriented Yarn) type and require an additional processing step (additional stretching or texturizing-stretching) in order for them to be regarded as finished and ready for use.
Also known is that at higher speeds, in excess of 5,500 meters/minute, it is possible to obtain some of the yarn characteristics sought, without stretching or spinning rollers, the orientation achieved at the die being already adequate to impart it with some satisfactory textile characteristics, i.e. low recovery and a suitable initial elastic modulus.
In general, however, one is liable to encounter considerable problems from two substantial standpoints: quality of the yarn and output rate. Under the former aspect, the problems are connected with the fact that the high operating tensions imposed by the high picking rate, do not permit a sufficient and regular number of interlacings to be obtained as required by modern high speed yarn processing methods and without prior re-twisting.
Furthermore, the need for imparting to the yarn a desired and necessary amount of finishing oil with the required regularity, and the fact that in processing PA6, for a correct formation of the packages, the yarn must be imparted with a substantial amount of moisture, creates a considerable problem of pollution of the working environment; therein, owing to the high speed of the yarn and the presence of thread cohesioning members (consisting of nozzles wherefrom high pressure air is ejected), atomizing of the added products (water and finishing agents) is quite high owing to the vibrations induced by the cohesioning fluid jet in the yarn. Such atomizing is highly harmful and undesired.
As to the output rates, single-stage processes at so high speeds, and in general all those (even at lower speeds) which are carried out without stretching rollers or other devices for reducing the tension on the thread during extrusion and picking, cause an extremely high number of breakages during the copping step, with considerable attendant waste production.
Furthermore, owing to the high operating tensions, the yarn interlacings are generally insufficient and irregularly distributed along the entire wound yarn.
Finally, the aforementioned difficulties of an irregular take-up of water finish result in the formation of irregular packages, which are consequently difficult to unwind in the course of subsequent processing steps.